Method for initializing an electro-optical material and system for capturing a fingerprint image

ABSTRACT

A fingerprint image capture system comprises an electro-optical material which captures a static fingerprint image and an apparatus for converting the static fingerprint image into an electronic signal. The image capture system includes an electrode for contacting a finger, and a bias supply for creating an electric field where epidermal ridges contact the electro-optical material. A transistor array senses charge or optical density variations in the electro-optical material to create an electronic representation of the fingerprint image. An initialization electrode places the electro-optical material into a uniform condition prior to acquisition of the fingerprint image. A physical artifact of the fingerprint image can be archived by removing the electro-optical material from the system following the fingerprint image acquisition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims benefit to,U.S. patent application Ser. No. 15/821942, filed Nov. 24, 2017,entitled “METHOD AND APPARATUS FOR CAPTURE OF A FINGERPRINT USING ANELECTRO-OPTICAL MATERIAL,” which is incorporated herein by reference inits entirety.

TECHNICAL FIELD

The present invention is related to the capture and retention of afingerprint image. More specifically, the present invention provides animproved method and apparatus for capturing a fingerprint whileachieving other useful objectives such as simplification of power supplyelectronics and non-volatile storage with a physical artifact forarchival.

BACKGROUND ART

Fingerprint capture devices are finding increased use in lawenforcement, security, financial, and other applications. The underlyingmotivation for fingerprint acquisition is the widely accepted beliefthat fingerprints are unique to an individual, and therefore comprise anexclusive attribute of identification. Biologically, fingerprintsconsist of raised portions of the epidermis, termed friction ridges orepidermal ridges, on the digits (fingers and toes), the palm of the handor the sole of the foot, comprising one or more connected ridge units offriction ridge skin. By virtue of this description, it should be evidentthat the term “fingerprint” can apply to several parts of the anatomywhich exhibit a uniquely defined pattern in the epidermis. Historically,fingerprint images were acquired for analysis and archival byapplication of ink to the skin, followed by making an impression of theskin onto a sheet of paper or cardstock.

The usefulness of a particular fingerprint or set of fingerprint imagesis intimately tied to their comparison and matching to a known set ordatabase of other fingerprint images. Prior to the availability ofcomputer-based storage systems, comparison and matching were performedmanually, and fingerprint images required physical archival. Thedevelopment of digital storage systems created an opportunity and needfor electronic means to acquire fingerprints so as to enable rapidstorage, distribution, and searching. A variety of techniques have beendeveloped for this purpose. The internet encyclopedia Wikipedia broadlygroups fingerprint readers into solid-state devices and optical readersutilizing one of several physical principles including optical,ultrasonic, capacitive or thermal means for detecting the differencebetween valleys and ridges on the skin.

In one example of a popular fingerprint acquisition system a LightEmitting Sensor film is used to create a visible image of a fingerprintby electrically exciting a light emitting phosphor using the ridges ofthe epidermis to couple an alternating current (ac) power source to thefilm. With reference to FIG. 1, a fingerprint acquisition system anddetection film detail of the prior art, a fingerprint acquisition system100 has capture and display device 110 showing a luminescent fingerprintimage 120. An acquisition electronics detail 130 includes skin epidermalridges 140 making an electrical contact to light emitting film 150 onthe surface of an image array sensor 160. An alternating current (to bedescribed infra) is coupled from the skin to the light emitting film150, which causes the film to glow at the points of contact, resultingin luminescent fingerprint image 120. The image array sensor 160 can becomprised of thin film transistors, cmos sensor pixels, or a ccd device.Skilled artisans will appreciate that the image sensor 160 isessentially similar to image capture devices widely employed in smartphone cameras, video capture devices, scanners, and digital x-raydetectors. The image capture process can employ electronics and softwarefor image acquisition well known to those skilled artisans. The lightemitting film 150 is detailed in light emitting film cross section 170,which illustrates the connection of an ac power supply 180 between a topelectrode and a phosphor layer within the film. In operation, thefingerprint acquisition system 100 is configured so that the topelectrode contacts a portion of the skin, making the epidermal ridgesinto an electrical potential node. An electrical current is inducedbetween the epidermal ridges and the phosphor layer, providing theenergy needed for the phosphor to glow.

The prior art fingerprint acquisition system of FIG. 1 has severalundesirable characteristics: First, the visible fingerprint image ispresent only for the duration that the finger is in contact with thelight emitting film and the ac power is applied. Absent a separateretrieval and display system, the fingerprint image cannot be examinedonce the finger has ceased contact with the device. Because thefingerprint image is constrained to only the portion of the fingercontacting the device, it is not possible to obtain an extended contactimage by rolling the finger from one side to another. The technique ofrolling the finger is commonly employed when acquiring fingerprintimages by means of ink and paper. These extended images are valuable formatching to forensic evidence which often includes incomplete or partialfingerprint images acquired from a variety of objects at crime scenes.Next, in order to effectively induce phosphor luminescence, the ac powersupply must be of relatively high frequency and high voltage. Typically,the electrical potential is about 80 to 250 volts or more at a frequencyof about 500 Hz to 40 kHz. The power supply needed to generate theseoperating conditions typically adds complexity, cost, and lowersefficiency as compared with a typical low voltage direct current (dc)power supply. Because the visible fingerprint image depends on lightemission from a phosphor, the image may be rendered difficult orimpossible to see under conditions of high external illumination, forexample outside in bright sunlight.

What is needed is an improved means for acquiring a fingerprint imagewhich combines simple low power electronics, image retention during andfollowing capture, and sunlight readability in a low cost system. Inparticular, a system which combines the best features of both electronicand traditional paper/ink based fingerprint capture would advance thestate of the art.

In order to better illustrate the features of the present invention,attention is now directed to FIG. 2, an example of an eletrophoreticdisplay material 200 known in the prior art. Electrophoretic materialsare a portion of a broad class of opto-electronic materials whichexhibit a change in visible appearance or optical characteristics inresponse to the application of an electric field. The electrophoreticdisplay material 200 typically has a transparent protective layer 210, atransparent electrode layer 220, with transparent micro-capsules 230 incontact with the transparent electrode layer 220. In one commercialimplementation, the transparent micro-capsules 230 are each about 50 to100 microns in diameter and contain positively charged white pigments240A and negatively charged black pigments 240B suspended in atransparent fluid 250. A free surface 260 of the electrophoretic displaymaterial is typically placed in contact with an array of pixelelectrodes, to be described infra. When an electric field is appliedbetween the transparent electrode layer 220 and the free surface 260,the corresponding charged pigments become visible at the transparentelectrode layer 220. The electric field is produced by applying apotential between the transparent electrode layer 220 and pixelelectrodes which are typically part of a thin film transistor (TFT)backplane to which the electrophoretic display material 200 islaminated. A first bias condition is produced by positively chargedpixel electrodes 270A. This first bias condition results in incidentwhite light illumination 280 having a white-colored response 290A. Asecond bias condition is produced by negatively charged pixel electrodes270B. This second bias condition results in incident white lightillumination 280 having a black-colored response 290B. The charge toeach capsule can also be bifurcated resulting in a half white half blacksurface. A combination of both positive and negative charges producesbifurcated pixel electrodes 270C. This bifurcated charge conditionresults in incident white light illumination having a gray-coloredresponse 290C.

Other variations of electrophoretic materials are known which produce asimilar optical response to applied electric fields. For example, in apresentation “Low Cost Flexible Displays Using Nanoscale Droplets” byMateusz Bryning, PhD a display material is described as comprisingcharged nanosize ink droplets dispersed in a porous matrix. In apresentation published on the world wide web, CLEARink Displaysdiscloses a electrophoretic display material in which charged particlesmodulate the response of the display material by disrupting a process oftotal internal reflection in the top plane of the display material.

SUMMARY OF THE INVENTION

The needs for an improved fingerprint acquisition device and system havebeen met in the present invention which presents a method and anapparatus for acquiring a fingerprint image by placing a finger intocontact with an opto-electronic material. In an exemplary embodiment ofthe present invention, the opto-electronic material is a pigmentedelectrophoretic material. The electrophoretic material is placed into aninitial condition by any of a plurality of means explained infra, sothat a first surface of the electrophoretic material is uniformlycolored, typically all white or all black. A first power supply node iscoupled to one side of the electrophoretic material, and a second powersupply node is coupled to the finger to be fingerprinted. When thefinger is brought into contact with the electrophoretic material on asurface opposing the first power supply node, the epidermal ridges actas electrodes and induce local movement of the pigment particles in theelectrophoretic material. As a result, an image of the fingerprint isgenerated in the electrophoretic material, with said image having astrong resemblance to a traditional ink on paper fingerprint.

When the finger being fingerprinted is removed from the surface of theeletrophoretic display material, the created image remains. If the biaspotential is removed or disconnected, there is no effect upon thefingerprint image. In this condition, the electrophoretic displaymaterial can be archived in a manner analogous to an ink on paperfingerprint. In the alternative, the fingerprint image can beincorporated into an electronic storage medium by any of several means.In one exemplary embodiment of the present invention, a conventionalCMOS (Complementary Metal Oxide Semiconductor) imaging device, wellknown to skilled artisans, can be employed to capture an electronicimage of the fingerprint image. In another exemplary embodiment of thepresent invention, the electrophoretic display material can be coupledto a thin film transistor (TFT) array which can read the fingerprintimage by optical or charge-sensing means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fingerprint acquisition system according to the prior art

FIG. 2 shows a diagram of an electrophoretic display material and itsresponse to an electric potential.

FIG. 3 is a pictorial representation showing a simplified system andmethod for acquisition of a fingerprint using an electrophoreticmaterial.

FIG. 4 is a pictorial representation of opposing surface images of anacquired fingerprint.

FIG. 5 is an illustration of a method for setting an initial conditionin an electrophoretic material.

FIG. 6 shows a method for fingerprint image capture by reflection of alight source from the image.

FIG. 7 shows a method for fingerprint image capture by transmission of alight source through the image.

FIG. 8A shows a pictorial representation and a schematic representationof a transistor array portion according to the prior art.

FIG. 8B shows two schematic variations for transistor array pixelconstruction according to the prior art.

FIG. 9 shows a transistor array configured to sense charge coupled to anelectrophoretic material.

FIG. 10 shows a transistor array configured to capture a fingerprintimage by passing light through an electrophoretic material.

FIG. 11 shows a block diagram for a fingerprint image capture system

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 3, an exemplary embodiment of the presentinvention comprises a fingerprint sampling apparatus 300. Finger 310 iscoupled to an electrode 320. A direct current (DC) bias supply 330having a positive potential and a negative potential is coupled to theelectrode 320 and to electrophoretic display material 200. In theexemplary embodiment, the positive potential of the direct current (DC)bias supply 330 is coupled to the electrode 320 and the negativepotential is coupled to the electrophoretic display material 200 bycoupling to the transparent electrode layer 220. A typical value fordirect current (DC) bias supply 330 is about 15 volts; a range ofpossible potentials for direct current (DC) bias supply 330 is betweenabout 1 volt and 30 volts. Inset 340 provides additional detail for theelectrophoretic display material 200 and its interaction with finger310. As shown in the inset 340, the electrophoretic display material 200comprises the free surface 260, transparent protective layer 210 andtransparent micro-capsules 230. Epidermal ridges 140 present on finger310 make contact with free surface 260 and the positive bias onelectrode 320 is coupled through finger 310 to the epidermal ridges 140.Transparent micro-capsules 230 are attracted to the points of epidermalcontact such that black pigments within the capsules are brought to freesurface 260. This alignment of the transparent micro-capsules 230results in the creation of a fingerprint image. The fingerprint image isstable unless deliberately altered; this stability will be discussedfurther infra. Because the acquired image is stable, it is possible toextend the fingerprint image by a side to side rolling action 350, in amanner completely analogous to the creation of a paper and inkfingerprint image. The ability to acquire these extended fingerprints isan important feature of the present invention.

Skilled artisans will appreciate that a variety of electro-opticalmaterials may be successfully employed for acquisition of thefingerprint image. The key attributes required for the electro-opticalmaterial are the ability to change optical appearance with theapplication of an electric field, and the ability to retain the imagefollowing termination of the finger contact. A specific example of amaterial meeting these requirements is an electrochromic material, suchas described by Switch Materials Inc. in a presentation on the WorldWide Web. In this instance, the electrochromic material can be inducedto change from a darkened state to a substantially transparent state byapplication of an electric field. The material can be returned to thedarkened state by removal of the electric field and illumination withillumination of sufficient intensity, such as daylight.

Additional detail of the fingerprint image is presented in FIG. 4 for anexemplary embodiment of the present invention in which theopto-electronic material is an electrophoretic display material. For thebias conditions outlined supra, the acquired fingerprint image willproduce a contact side fingerprint image 410 having a white backgroundwith black lines corresponding to the points of contact between thefinger epidermal ridges and the electrophoretic display material 200.The construction of the electrophoretic display material 200 is suchthat on the side opposite to contact side, an opposing side fingerprintimage 420 will be formed. The opposing side fingerprint image will be anegative mirror image of contact side fingerprint image 410. Skilledartisans will appreciate that other opto-electronic materials may notproduce the opposing side fingerprint image 420. For example,electrophoretic materials which alter appearance based on a change ininternal reflection will evidence a fingerprint image on only onesurface. Other electro-optic materials, for example an electrochromicmaterial, will produce an image which is substantially uniform on bothsides of the material, excepting for a mirroring of the image.

Prior to the acquisition of a fingerprint, the electrophoretic displaymaterial 200 must be initialized into a condition of uniform appearance.Additionally, once acquired a fingerprint image will be stable until theelectrophoretic material is reinitialized. With reference to FIG. 5, inan exemplary embodiment of the present invention an initializationmethod 500 comprises a flexible electrode film 510 placed into contactwith the free surface 260 of the electrophoretic display material 200having a region of unaligned pigments 520. The direct current (DC) biassupply 330 is coupled to the flexible electrode film 510 and to thetransparent electrode layer 220. As contact is initiated between theflexible electrode film 510 and the free surface 260, transparentmicro-capsules 230 align according to the polarity of the applied bias,producing a region of uniformly aligned pigments 530. In FIG. 5, thepolarity of the direct current (DC) bias supply 330 is shown configuredto cause the transparent micro-capsules 230 to align such that the whitepigments are presented to the free surface 260. Reversing the polarityof the direct current (DC) bias supply 330 would conversely cause theblack pigments to be presented to the free surface 260. Initializationis complete when the entire free surface 260 has been brought intocontact with the flexible electrode film 510. Once initialization hasbeen completed, the flexible electrode film 510 can be retracted and thedirect current (DC) bias supply can be removed without changing theappearance of the electrophoretic display material 200. Theelectrophorectic display material 200 is now ready for a new fingerprintacquisition.

Once a fingerprint image has been captured, a plurality of methods areavailable for the purpose of storage and archival of the image. In afirst exemplary embodiment of the present invention, the electrophoreticdisplay material 200 is simply disconnected from direct current (DC)bias supply 330. With the direct current (DC) bias supply 330 removed,the fingerprint image will remain stable on the electrophoretic displaymaterial 200. Skilled artisans will appreciate that it would be a verysimple matter to make a small piece of electrophoretic display material200 removable from fingerprint sampling apparatus 300, producing astable artifact which could be shared and archived much like apaper-and-ink fingerprint. Pre-initialized coupons of electrophoreticdisplay material 200 could be replaced into fingerprint samplingapparatus 300, enabling capture of additional fingerprints, eachfingerpint having its own stable artifact in the form of a dedicatedpiece of electrophoretic display material 200.

In a second exemplary embodiment of the present invention, thefingerprint image is captured by means of a conventional imagingapparatus. With reference to FIG. 6, reflective fingerprint imagecapture system 600 comprises an illumination source 610 producingillumination 620 which impinges on contact side fingerprint image 410present on electrophoretic display material 200. Illumination 620 isreflected back to imager 630 which captures the fingerprint image by anof a plurality of methods know to skilled artisans. For example, imager630 may be a CMOS imager of the kind frequently found in smart phones.Alternately, imager 630 could be a CCD device, or other imaging array.Imager 630 may even comprise a conventional camera utilizinglight-sensitive chemical emulsion film. Once the fingerprint image hasbeen captured by imager 630, the image can be stored and shared byelectronic means well known to skilled artisans; the required technologyis well known in the typical smartphone. Skilled artisans willappreciate that is also possible to configure the capture system 600 soas to acquire the opposing side fingerprint image 420. Thisconfiguration is not shown to avoid obscuring the present invention.

A third exemplary embodiment of present invention appears in FIG. 7 astransmissive fingerprint image capture system 700. In this embodiment,illumination source 610 and imager 630 are arranged so as to beseparated by electrophoretic display material 200. Illumination 620 istransmitted through electrophoretic display material 200, conveyingopposing side fingerprint image 420 to imager 630. Skilled artisans willappreciate that numerous variations are possible for illumination source610 and imager 630. For example, in some instances ambient illuminationmay be sufficient to obviate the need for a dedicated source.Alternately, illumination source 610 could be a visible light emittingdiode (LED), an incandescent lamp, or other luminescent material. Theillumination source need not be discrete; edge-illumination techniquesare well known to those skilled in the art and are frequently employedin commercial display devices. If it were desirable to limit visiblelight radiation, illumination source 610 could be an infrared-emittingLED, or other wavelength-specific LED matched to the characteristics ofthe imager 630.

Additional embodiments of the present invention use a transistor arrayto store the captured fingerprint image electronically. Transistorarrays are widely known to skilled artisans for their use in CMOSimagers, electronic x-ray detectors, television and smartphone displays,and scientific instruments. To better illustrate the application of atransistor array in the present invention, attention is directed to FIG.8A, an illustration of a portion of a transistor array known in theprior art. Transistor array portion 800A typically comprises an array ofpixel elements fabricated on a substrate using any of a pluralitymethods known to artisans skilled in semiconductor device fabrication.Common fabrication techniques include CMOS devices fabricated oncrystalline silicon wafers and thin film transistor (TFT) arraysfabricated on glass or flexible substrates. The CMOS transistor arraysare often employed for digital image capture devices such as cameras andhigh resolution dental x-rays. The thin film transistor arrays arefrequently found in display and television screen applications. [Thinfilm transistor arrays can be fabricated by a large number of techniquesusing materials such as low temperature poly-silicon (LTPS), amorphoussilicon (aSi), organic semiconductors, and mixed metal oxides such asindium gallium zinc oxide (IGZO). Further, those thin film transistorarrays can be produced on a wide variety of substrates such as glass,flexible polymers, and even paper. Therefore, the usage of the term thinfilm transistor array with respect to the present invention should beunderstood as not being limited to a specific transistor materialchemistry or substrate.] Transistor array portion 800A includes a pixelelectrode 810 and an inactive zone 820. The actual number of pixels intransistor array portion 800A can be varied by changing the number ofrows and columns to be fabricated using methods well known to skilledartisans. Array sizes can be quite large, having many millions ofpixels.

The electrical operation of transistor array portion 800A can beunderstood with reference to transistor array portion schematic 800Billustrating how elements of the array are interconnected. The arrayportion comprises gate lines G1, G2, and G3 running parallel in a firstdirection, and data lines D1, D2, and D3 running parallel so as to beorthogonal to gate lines G1, G2, and G3. Each pixel includes atransistor 830 having a gate node 1 coupled to an array gate line, and asource node 2 coupled to an array data line. The drain node oftransistor 830 is coupled to pixel electrode 810. The data lines D1, D2,and D3 are each coupled to a charge-sense amplifier 840. The chargesense amplifier 840 measures the charge present on its respective dataline and converts that charge measurement to an electrical potential.Charge sense amplifier 840 may be coupled to an analog to digitalconverter (ADC) circuit to create a digital representation of theelectrical charge measurement. The ADC circuit is not shown to avoidobscuring the present invention. Skilled artisans are familiar withmethods for applying time-varying electrical potentials to the gatelines and data lines so as to be able to individually address individualpixels and measure the charge of each. The collection of measurementscan be combined to yield a conventional digital image representation ofthe array configuration.

The pixel construction may be varied according to the intended use andpurpose of the transistor array. With reference to FIG. 8B, a firstpixel design variation 850 includes a capacitor 860 coupled to a drainnode 3 of transistor 830. The capacitor 860 is also typically coupled toa ground potential. The capacitor 860 stores charge, holding a potentialon pixel electrode 810 constant when transistor 830 is biased so as toelectrically decouple pixel electrode 810 from the gate and sourcelines. This can be useful in display application when it is desirable tomaintain a constant pixel state without continuously providing a voltagebias by maintaining transistor 830 in an electrically conductive state.With further reference to FIG. 8B, a second pixel design variation 870includes a photodiode 880 coupled to drain node 3 of transistor 830.Photodiode 880 is coupled to bias connection 890 which provides a commonelectrical node for setting a voltage potential on all photodiodescomprising the transistor array. The photodiode 880 is light sensitiveand is typically fabricated on an upper surface of the pixel. Whenappropriately biased using techniques known to skilled artisans,photodiode 880 is capable of providing a quantity of electrical chargeproportional to the degree of illumination on the pixel.

An exemplary use of a transistor array to capture a fingerprint image bymeans of sensing charge appears in FIG. 9. Charge sense configuration900A comprises contact side fingerprint image 410 coupled to transistorarray 910, configured to sense pixel charge. Charge sense configurationcross section 900B further illustrates the operation of the system.Contact side fingerprint image 410 is comprised of transparentmicro-capsules 230 near free surface 260. The transparent micro-capsules230 each possess a small amount of electrical charge, analogous to anelectrical capacitor. The orientation of the transparent micro-capsulesdetermines the polarity and quantity of charge which is presented tofree surface 260. The free surface 260 is coupled to pixel electrodes810, which are in turn coupled to transistors 830. When positivelybiased, gate line GX places transistors 830 into a conductive condition.The electrical charge possessed by transparent micro-capsules 230 istransferred through transistors 830 to the charge sense amplifier 840.The charge sense amplifer 840 produces a voltage output which isproportional to the degree of charge transferred. Hence, the voltageoutput provides an indication of the fingerprint image as represented bythe condition of the transparent micro-capsules 230.

In yet another exemplary embodiment of the present invention, the use ofa transistor array to capture a fingerprint image optically is presentedin FIG. 10. Transistor array 1000, having pixels configured withphotodiodes, is coupled to opposing side fingerprint image 420 ofelectrophoretic display material 200. Illumination source 610 providesillumination 620 which passes through electrophoretic display material200 onto the pixels of transistor array 1000. Each pixel produces acharge output proportional to the degree of illumination, by meansalready discussed supra. The individual pixel responses are accumulatedand organized to produce the fingerprint image.

A system for the acquisition and storage of fingerprints according tothe present invention appears in FIG. 11. Improved fingerprintacquisition system 1100 comprises electro-optical material 1110 coupledto transistor array 910. Array addressing electronics 1120 provideselectrical controllable electrical connections to gate and drain linesof transistor array 910. Direct current (DC) bias supply 330 provides acontrollable electrical potential to flexible electrode film 510 andfinger contact electrode 320. Direct current (DC) bias supply 330 may beconfigured to provide a plurality of electrical potentials in order toprovide multiple control voltages required by the system. The directcurrent (DC) bias supply 330 is also coupled to the array addressingelectronics 1120 to provide suitable electrical potentials to the gateand drain lines of transistor array 910. The drain lines of transistorarray 910 are coupled to charge sense amplifiers 840 which providevoltage potentials proportional to the charge present on the drain linesof transistor array 910. The charge sense amplifers 840 are coupled toanalog to digital (A2D) conversion circuits 1130 which convert thevoltage potential outputs of the charge sense amplifiers 840 intodigital representations which can be interpreted by a computer. Theoutputs of the analog to digital (A2D) conversion circuits are fed to asystem control computer 1140 which performs the tasks of system control,image formation, and image storage. The illumination source 610 can beoptionally included in the system for the purpose of observing theacquired fingerprint image. In an alternate embodiment of the presentinvention, improved fingerprint acquisition system 1100 can beconfigured to capture the fingerprint image optically, using transistorarray 1000 having photodiode pixels. In this instance, the illuminationsource 610 provides the illumination necessary to convey the fingerprintimage into transistor array 1000.

In the foregoing specification, the present invention has been describedwith reference to specific embodiments thereof. It will, however, beevident to a skilled artisan that various modifications and changes canbe made thereto without departing from the broader spirit and scope ofthe present invention as set forth in the appended claims. For example,although the apparatus and method of the present invention is describedprimarily in reference to the acquisition of fingerprints, skilledartisans will appreciate that the present invention may be applied tofootprints, palm prints, or to any portion of the anatomy havingepidermal ridges. Additionally, although the present invention isdescribed with reference to human anatomy, those same skilled artisanswill recognize that the present invention can be applied to other animalspecies which possess anatomical features appropriate to imaging.Additionally, although multiple methods of storing the fingerprint imagecaptured by means of electrophoretic display material have beendescribed, skilled artisans will recognize that additional storagemethods are possible. Finally, skilled artisans will also recognize thatalthough the present invention is described in association withelectrophoretic materials typically employed for display applications,that customized electrophoretic materials are possible to further thescope and utility of the present invention. Additionally, othermaterials which exhibit a change in optical properties as a function ofelectric field may be utilized in order to capture the fingerprintimage. The specification and drawings are, accordingly, to be regardedin an illustrative rather than a restrictive sense.

What is claimed is:
 1. A method for initializing an electro-opticalmaterial prior to acquiring a fingerprint image using theelectro-optical material, the method comprising: coupling a first biassupply terminal to an electrode layer on a first side of theelectro-optical material; coupling a second bias supply terminal to aninitialization electrode which is mechanically brought into contact witha second side of the electro-optical material, the second side beingopposed to the first side; and coupling a direct current bias supply tothe first bias supply terminal and the second bias supply terminal. 2.The method of claim 1, wherein the initialization electrode is aflexible conductive film.
 3. The method of claim 1, wherein theelectro-optical material is an electrophoretic display material.
 4. Amethod for initializing an electro-optical material prior to acquiring afingerprint image using the electro-optical material, the methodcomprising: illuminating the electro-optical material while theelectro-optical material is electrically floating and able to reach auniform electrical potential throughout its volume.
 5. The method ofclaim 4 wherein the electro-optical material is an electrochromicmaterial.
 6. A system for capturing a fingerprint image, comprising: anelectro-optical material, said electro-optical material beingcharacterized by producing a change in optical properties in response toan applied electric field; a bias supply capable of supplying a directcurrent potential to a first electrode of the electro-optical material;a flexible initialization electrode coupled to the bias supply andcapable of contacting the electro-optical material while applying anelectrical bias to bring the electro-optical material into a conditionof uniform appearance; a finger contact electrode coupled to the biassupply and configured to induce an electric potential on a finger whilethe finger is in contact with the electro-optical material; a thin filmtransistor array having gate lines, drain lines, and photodiodes, saidtransistor array configured to convert an optical signal incident upon apixel into an electrical charge, the transistor array mechanicallycoupled to the electro-optical material during an electronic capture ofa fingerprint image; array addressing circuits configured to applyelectrical control potentials to gate and drain lines in the thin filmtransistor array; charge sense circuits coupled to the thin filmtransistor array through the array addressing circuits, the charge sensecircuits configured to produce an electrical potential in proportion tothe amount of charge present on a drain line of the thin film transistorarray; analog to digital conversion circuits coupled to the charge sensecircuits, the analog to digital conversion circuits providing a digitalrepresentation of the electrical potentials produced by the charge sensecircuit; an illumination source configured to pass light through afingerprint image present in the electro-optical material, the lightemerging onto the thin film transistor array; and a microprocessor-basedcontrol system which coordinates system control functions for biascontrol, array addressing, illumination, charge sense and digitalconversion, image processing and storage.
 7. The system of claim 6wherein the electro-optical material is an electrophoretic materialcapable of being removed and physically archived after the fingerprintimage capture is complete.
 8. The system of claim 6 wherein theillumination source operates at a wavelength invisible to the human eye.